Olefins such as ethylene and propylene are produced by causing a material fluid of hydrocarbon (such as naphtha, natural gas or ethane) to flow at a high velocity through a thermal cracking tube heated from outside and heating the material fluid to a reaction temperature range for thermal cracking.
To effect the thermal cracking reaction efficiently, it is important that the material fluid to be passed at a high velocity through the tube be heated to the thermal cracking temperature range throughout the tube channel to the radially central portion thereof, with overheating avoided to the greatest possible extent. If the material fluid is overheated, hydrocarbons are excessively converted to light fractions (formation of methane, free carbon, etc.) or cracked products will be subjected to polycondensation reaction to result in a greater reduction in the yield of the desired product. Overheating also results in promoted coking (deposition of free carbon on the tube inner surface) to give a lower heat transfer coefficient to the tube, entailing a need for frequent decoking work.
Accordingly, it is practice to provide on the inner surface of the thermal cracking tube projections serving to agitate the fluid flow, such that the agitation by the projections ensures improved heat transfer to the fluid flowing at a high velocity for a rise in the temperature by rapid heating (see, for sample, Patent Literature 1). Consequently, the reaction terminates within a shorter period of time to avoid the conversion to light fractions due to overheating. Furthermore, the improved heat transfer efficiency of the thermal cracking tube makes it possible to heat the thermal cracking tube at a lower temperature to entail the advantage of extending the service life of the cracking tube.
Patent Literature 1 discloses a thermal cracking tube wherein a region having no projections extends over the entire length of the tube in parallel to the axis thereof to assure a reduced pressure loss without impairing the heat transfer efficiency of the tube.
Patent Literature 1: Republished Patent WO2004-046277